1. Field of the Invention
This invention relates to an ultrathin solid membrane, a process for production thereof, and use thereof for concentrating a specified gas such as oxygen gas in a gaseous mixture such as air.
2. Field of the Invention
For some years, there has been a marked advance in membrane-dependent separation techniques in various fields. A technique for separating a gaseous mixture using a membrane, however, is a relatively recent technological development. The technical difficulty of separating a specified gas from a gaseous mixture lies in the development of a material capable of permitting permeation of the specified gas with sufficient selectivity and at a sufficient permeating speed and also in the establishment of a technique for forming such a material into a very thin membrane having a uniform thickness and a large area.
Generally, the amount of a gas permeating a homogeneous membrane is defined by the following equation. EQU X=P.times.(P.sub.1 -P.sub.2).times.A/l
wherein
X represents the permeating velocity [cc(STP)/sec.] of the gas, PA1 P is the permeation coefficient [cc(STP)x.multidot.cm/cm.sup.2 .multidot.cmHg.sec] of the gas, PA1 (P.sub.1 -P.sub.2) is the difference of the partial pressures (cmHg) of the gas on both surfaces of the membrane, PA1 A represents the area (cm.sup.2) of the membrane, and PA1 l is the thickness (cm) of the membrane.
It is clear therefore that once the material of which the membrane is made and a gas to be permeated are specified, the amount of permeation of the gas depends upon the thickness and area of the membrane. It is desired to make the membrane thickness as small as possible and the area of the membrane as large as possible.
As a prior attempt to produce a membrane having a small thickness and a large area, there is known a process for producing an ultrathin membrane by a batchwise technique, which comprises dropping a solution of a blend of a methylpentene polymer and an organopolysiloxane-polycarbonate copolymer in a solvent onto the surface of water to allow the solution to spread spontaneously on the surface of water (see U.S. Pat. No. 4,192,842). As described in the specification of this Patent, the use of the organopolysiloxane-polycarbonate copolymer makes it possible for the solution to spread spontaneously on the surface of water. The specification of this U.S. patent also describes a method involving the use of a solution of the methylpentene polymer alone in a solvent. Investigations of the present inventors, however, have shown that the method using such a solution which does not contain the organopolysiloxane-polycarbonate copolymer did not lead to the successful production of an ultrathin membrane having a uniform thickness and a large area. Thus, although the method disclosed in the above cited U.S. Pat. No. 4,192,842 involving the use of a solvent solution of the methylpentene polymer alone is one attempt at the production of an ultrathin membrane, it is at least not easy to produce an ultrathin membrane having a uniform thickness and a broad area suitable for practical application.
U.S. Pat. No. 4,132,824 from which the above-cited U.S. Pat. No. 4,192,842 was divided out claims only an ultrathin membrane comprising a blend of a methylpentene polymer and an organopolysiloxane-polycarbonate copolymer.
U.S. Pat. No. 4,155,793 discloses a process for continuously producing a composite laminar membrane consisting of a web and two thin polymeric films held thereon in the superimposed state, which comprises feeding solvent solutions of polymer into two wells provided in opposing directions on the surface of an aqueous medium to spread the solvent solutions on the surface of the aqueous medium, and continuously feeding a web coming into the aqueous medium at a position intermediate between the two wells thereby to hold on the web two thin films formed from the solvent solutions spread on the water surface. This process is characteristic in that two thin films are continuously formed on a stationary aqueous medium, and these two films are simultaneously held on one web and continuously recovered from the surface of the stationary aqueous medium.
The ultrathin film produced by such a process is used generally for producing an oxygen-enriched gaseous mixture from air, as is disclosed in the specification of the above-cited U.S. Patent. Such a use is embodied as a device for obtaining an oxygen-enriched gas from air in the specifications of U.S. Pat. Nos. 3,976,451 and 4,174,955.
It is an object of this invention therefore to provide a process for producing from a hydrocarbon addition polymer a very thin membrane having a uniform thickness and a substantially equivalent gas separation factor to the inherent gas separation factor of the addition polymer.
Another object of this invention is to provide a process which comprises dissolving a hydrocarbon addition polymer in an organic solvent which has a suitable surface tension with respect to water to form a solvent solution of the polymer which has a suitable surface tension with respect to water, whereby a very thin solid membrane having a uniform thickness and the desired gas separation factor is formed on the surface of water.
Still another object of this invention is to provide an ultrathin solid membrane having a uniform thickness, the desired gas separation factor and a large area.
Yet another object of this invention is to provide a process for continuously producing an ultrathin solid membrane having the above properties by carrying it on a porous sheet-like material.
A further object of this invention is to provide use of an ultrathin solid membrane having the above properties for the production of a gas containing a specified component gas (e.g., oxygen gas) concentrated from a gaseous mixture (e.g., air) of two or more gases.
An additional object of this invention is to provide a module used to concentrate a certain gas by using a solid membrane having the aforesaid properties, and an oxygen enricher comprising the aforesaid module for producing an oxygen-enriched gas from the air.
Other objects and advantages of this invention are apparent from the following description.